1. Field of the Invention
This invention relates to light sensitive thermally developable (i.e., photothermographic), imageable systems, particularly those employing an oxidizer, sensitizing dye, and a leuco dye. More particularly, this invention relates to imageable systems comprised of a leuco dye oxidizing acid salt and a sensitizing dye.
2. Description of the Related Art
Many processes and compositions use leuco dyes to provide optical densities in the imaged article. For example, U.S. Pat. No. 4,017,313 uses a combination of a photosensitive leuco dye, a photosensitizer for the dye, an aromatic aldehyde and a secondary or tertiary amine. Other photosensitive systems using leuco dyes are included in U.S. Pat. Nos. 3,390,997, 2,884,326, and 2,772,284. The mechanism of these last two patents is disclosed in "Aromatic Aldehyde-Leuco Dye Photooxidation" H. D. Hartzler, Pure Appl. Chem. 1979, 49, pp. 353-356. Light-Sensitive Systems (J. Kosar; John Wiley and Sons: New York, 1965; p. 369), describes print-out photosensitive systems comprising a binder, leuco dye, organic halogen compound and a photosensitizing dye. Because these are printout systems, there is no thermal amplification. A great many photosensitive materials have been used in different imageable processes utilizing various photoinitiated phenomena such as photohardening of polymerizable materials (e.g., negative-acting printing plates, photosolubilizing materials (e.g., positive-acting printing plates), light initiated diazonium salt coupling reactions (e.g., diazonium microfilm), etc. A class of iodonium photoinitiators for both cationic and epoxy polymerization (e.g., U.S. Pat. Nos. 4,026,705 and 3,981,897) has also been proposed as equivalent to other photoinitiators in certain ethylenically unsaturated printing plate compositions (e.g., U.S. Pat. No. 3,741,769).
Photothermographic imageable systems are well known in the art. By definition photothermographic systems are light sensitive imageable systems which are thermally developed. Photothermographic systems typically require development temperatures in the range of 80.degree. to 200.degree. C. A number of imageable systems employ photosensitive compounds, leuco dyes or bleachable dyes, and nitrate salts to generate color images, imageable systems, which are sensitive to ultraviolet (UV) light, comprising a leuco dye or bleachable dye, nitrate ion, and diazonium salts in a binder are disclosed in U.S. Pat. No. 4,370,401. In those cases wherein a leuco dye system is employed, a photothermographic, negative acting imageable system is provided; that is, the optical density in the final image is more dense in areas which are light struck than in areas which are not light struck. Conversely, in those cases wherein a bleachable dye system is employed, a photothermographic, positive acting imageable system is provided, that is, the optical density in the final image is more dense in areas which are not light struck than in areas which are light struck.
Related imageable compositions comprising a diazonium salt and leuco dye in a binder is disclosed in U.S. Pat. No. 4,394,433. These unamplified compositions are positive-acting photothermographic compositions, and differ fundamentally from the compositions of the present invention, which are amplified by the action of an oxidizing anion.
Additional light sensitive, thermally developable imageable systems are known. U.S. Pat. No. 4,460,677 describes a thermally developable imageable system comprising a leuco dye, nitrate ion, and a spectrally sensitized organic compound having photolyzable halogen atoms. Similarly, U.S. Pat. No. 4,386,154 describes a thermally developable imageable system comprising a leuco dye, a nitrate ion, and a spectrally sensitized compound selected from (1) aromatic iodonium salts and (2) compounds containing photolyzable halogen atoms. Both of these compositions act as a negative image forming systems in that, upon heat development, the greatest image density is formed in the light struck areas.
Light sensitive, thermally developable imageable systems are also described in several Japanese Patents. For example, Japanese Pat. No. 77,025,330 pertains to a UV light sensitive two component positive acting imageable composition comprised of an oxazine or phenothiazine leuco dye (BLMB) mono or disubstituted with a dialkylamino group, and an oxidizing agent such as nitrate ion.
Japanese Pat. No. 77,004,180 describes the use of triplet sensitizers for BLMB. Suitable sensitizers are aromatic carbonyl compounds and aromatic nitro compounds. Said patent describes both negative and positive systems, and is a counterpart to Japanese Pat. No. 77,025,330. The compositions described therein are UV light sensitive whereas the compositions of this invention are light sensitive through the entire wavelength range of 300-900 nm.
Japanese Pat. No. 76,035,847 describes photosensitive heat fixable recording materials containing a free radical producing organic halogen compound, leuco dye and a base. This is a negative acting system which contains no oxidizer.
Japanese Pat. No. 77,025,088 describes photosensitive compositions containing an acid sensitive leuco dye (e.g. naphthospiropyran), a photochemical acid generating agent which is a mixture of an organic halide (e.g., CBr.sub.4), with a furan containing compound.
Japanese Pat. No. 79,001,453 describes a photothermographic material which contains an oxidizer, a compound with the oxidizer to change or develop color, and a compound which deactivates the color developer either in exposed or unexposed regions. Images can be either positive or negative. The light-sensitive materials used were colorless or nearly colorless aryl quinones and ultraviolet light sources were used. No other photosensitive materials appear to be taught.
Decolorizable imageable systems comprising a binder, nitrate salt, acid, and dyes are disclosed in U.S. Pat. Nos. 4,336,323 and 4,373,020. These systems are particularly useful as antihalation layers in photothermographic systems where the development temperature acts to bleach the dye.
The use of photobleachable dyes including o-nitroarylidene dyes as antihalation or acutance dyes is known in the art: U.S. Pat. Nos. 4,111,699; 4,271,263; 4,088,497; 4,033,948; 4,028,113; 3,988,156; 3,988,154; 3,984,248; 3,615,432 (RE28,225).
Japanese Pat. No. 88,058,108 teaches the use of salts of heptavalent and hexavalent molybdenum compounds and benzoyl leuco methylene blue in thermally imaged compositions. Also included are nitric acid salts of leuco methylene blue, but only when simultaneously in the form of a molybdenum complex or salt. Nitric acid, sulfuric acid, and hydrochloric acid are taught as forming useful salts with leuco methylene blue in the presence of molybdenum compounds, only nitrate salts are useful in the present invention.
Japanese Pat. No. 88,058,109 teaches the use of salts of heptavalent molybdenum compounds and benzoyl leuco methylene blue in thermally imaged compositions. Also required is an included compound containing both oxidizer and reducer moieties.
Japanese Pat. No. 88,058,110 describes the use of salts of hexavalent and heptavalent molybdenum compounds condensed to form ios- and hetero-polyacids in photothermally imaged compositions.
Japanese Pat. No. 88,058,111 discloses the use of salts of hexavalent and heptavalent molybdenum compounds in the presence of blocked phenols in thermally imaged compositions.
The latter four Japanese references apparently show that only blue to black images were successfully generated. This is expected since molybdic acid oxidation of organic compounds generally leads to dark blue-black color formation. This fact makes molybdenum based oxidizers unsuitable for applications in which other colors such as red or green are desired. Imaging systems free of molybdenum active ingredients are therefore desired.